Updating to latest official Polymer TodoMVC implementation

As Polymer is a bit of a moving target, the underlying library has changed quite a lot since the last time we added it to master. This PR brings the app back in line with [Polymer/todomvc](https://github.com/Polymer/todomvc).

Please pardon the long set of diffs. Polymer has also heavily changed since the last updated the version we used from Bower.

Let me know if you have any questions! Would ideally like to land/merge this before we release this week.